Combined hammer mill crushing and oversize particle separating apparatus



r H Y mm 2m m A 5 5,? MM 2 1 e m s 5 W. M. SHELDON COMBINED HAMMER MILL CRUSHING AND OVERSIZE PARTICLE SEPARATING APPARATUS IIIIIIII I/IIIII/ May 15, 1951 Filed June 29, 1944 May 15, 1951 w. M. SHELDON 2,552,596

COMBINED HAMMER MILL CRUSHING AND OVERSIZE PARTICLE SEPARATING APPARATUS Filed June 29, 1944 s Sheets-Sheet 2 IN VEN TOR.

May 15, 1951 SHELDON 2,552,596

W. M. COMBINED HAMMER MILL CRUSHING AND OVERSIZE PARTICLE SEPARATING APPARATUS Filed June 29, 1944 3 Sheets-Sheet 5 W V a i Q.

VJIIIIIIIII- V TOR.

IN WWW Patented May 15, 1951 UNITED STATES PATENT OFFICE COMBINED HAMMER MILL CRUSHING AND OVERSIZE PARTICLE SEPARATING AP- PARATUS William M. Sheldon, Elizabeth, N. 3., assignor t Pulverizing Machinery Company,

Summit,

14 Claims.

1 This invention relates to an apparatus for pulverizing various materials. The general object of the invention is to provide an improved .pulverizing mill which will produce pulverized material of extreme fineness almost entirely free from oversize particles. The invention aims also to provide such a pulverizing mill which does not become clogged by accumulation of ground material in corners or against dead surfaces, and which is of comparatively high efficiency, of compact and sturdy construction, and of high capacity for its size. Other objects and advantages of the invention will appear from the following description.

In this invention the pulverized and partly pulverized material, that is pulverized material of different degrees of fineness, as it escapes from the boaters of the high speed rotor of an impact pulverizing mill is received by and subjected to the diffusing action of an inflowing current of air, and the diffused material is carried by the air current inwardly and radially beneath the heaters or hammers where the air current divides, part passing radially inwards through a separator wheel, while the remainder flows outwards for re-entry into the hammer. The separator wheel usually rotates in unison with and is, in effect, a part of the rotor which carries the hammers.

The separator wheel has a series of radially arranged passages therethrough closed on all sides either by portions of the wheel or the cas ing of the mill but open top and bottom. Means are provided for producing a lower pressure on the discharge side or inner periphery of the separator wheel than on the inlet or hammer chamber side to cause air to flow radially inwards through such passages. As the passages are closed on all sides the air flowing through them is given the same rotative speed as the separator wheel. This rotation produces centrifugal force on the air so that the pressure difference must be great enough to overcome such force as well as those of friction and the like.

The air flowing radially through the separator carries with it material in suspension. Such material, like the air by which it is carried, is given a rotative speed about the rotor axis equal to that of the separator wheel. This rotation produces centrifugal forces which cause the particles to move radially outwards through the air in which they are suspended with speeds which vary with the size of the particles, the large particles moving faster than the small ones. This variable velocity movement of the particles is in the opposite direction to the constant velocity air movement so that the resultant movement is outwards for the large particles and inwards for the small. This means that by suitable proportioning the rate of rotation of the separator wheel and the rate of air flow all particles over a predetermined size can be rejected and returned to the boaters or hammers.

Since the passages in the separator wheels form the only outlets for the air in the hammer chamber there can be no by-passing of such passages and hence all of the air leaving the hammer chamber has imparted to it substantially the same rotative and radial velocity. From this it follows that all the pulverized and partly pulverized material in such air is necessarily subjected to a substantially uniformly operatin outward centrifugal force working against a substantially uniformly operating radial inward air drag.

The high centrifugal force to which all material is subjected before it can leave the hammer chamber enables the rate of air flow to be increased to a high figure before unacceptably coarse material is swept out of the chamber. This is important, especially in very fine grinding, since high capacity and efficiency both require the removal of acceptably fine material as soon after it is produced as possible.

For similar reasons the path from the hammers to the separator wheel and back to the hammers should be as short as practicable. For these reasons the invention includes the provision of an annular hammer chamber of small cross-section surrounding the hammers fairly closely on all sides. Likewise the outer peripheral area of the hammer path, which is roughly a measure of the pulverizin capacity of a mill, should be large in comparison with the volume of the hammer chamber. Otherwise there will be an unduly large amount of air carrying material in suspension uselessly circulating around in the hammer chamber.

A full understandin of the invention can best be given by a detailed description of a mill embodying the apparatus features of the invention in the form now considered best and adapted for carrying out the invention, and of the operation thereof. Such a. description will now be given in connection with the accompanying drawings illustrating such a mill. In said drawings:

Fig. 1 is a side view of the mill partly in central vertical longitudinal section;

Fig. 2 is a sectional view taken on line 2--2 of Fig. 1 with parts shown in elevation;

Fig. 3 is a sectional view taken on line 33 of Fig. 1 with the rotor and the separator wheel shown partly in elevation and partly broken away;

Fig. 4 is a fragmentary sectional view showing a modified construction of the rotor and separator wheels;

Fig. 5 is a plan view of a mill having a single outside suction fan;

Fig. 6 is a sectional view taken on line 6 of Fig. 5 and Fig. 7 is a detail view showing the direction of flow of air into, through and out of the hammer chamber.

Referring to the drawings, and first to Figs. 1, 2 and 3, the mill housing comprises a central or main portion It providing a. cylindrical mill chamber ll having an imperforate peripheral wall, two end portions i2 providing, together with two partition and spacing members l3, two fan Chambers !4. A shaft l5 mounted in bearings It at either end of the housing and which may be driven by any suitable means extends through the housing. The bearings it are mounted on cross members of the base frame [8. Mounted on and keyed to the shaft :5 is a rotor having a body part formed by two body discs 28 seated against shoulders 2| on the shaft and a plurality of beater blades 22 carried by arms 23 the inner bossed ends of which extend between the discs and are pivotally connected thereto,

When the mill is in operation the beater blades extend radially from the body discs 20, their outeredges being close to the cylindrical peripheral wall of the chamber as usual in hammer mills. The peripheral wall of the chamber is most desirably provided with a liner 24 having a transversely corrugated inner face as shown.

Also mounted on the shaft I5 in the mill chamber are two separator wheels, one on each side of the rotor, each comprising a plurality of circumferentially spaced radial blades 25 secured to a disc 26 extendin against one of the rotor body discs 20 from a hub 27. The outer side edges of the blades 25 are most desirably not connected by a shroud plate'or otherwise, but the blades extend with their free outer side edges just clear of an annular portion 28 of. the end wall of the chamber. The diameter of the separator wheels is substantially less than that of the rotor, the open material-receiving periphery of each separtor wheel being radially in line with and spaced inwardly from the beater blades so as to leave a comparatively wide flow space between'them. The blades are relatively short radially, leaving an open flow space between their inner edges and the hub 21.

' Instead of the blades of the separator wheels being, as in Fig. 1, carried by discs 26 on hubs thus constituting separately formed and separately removable wheels, the blades might be carried by the rotor body discs, as shown by Fig. 4, the rotor discs 20 then being of sufiicient diameter to serve as the discs of the separator wheels as well as formin the beater carrying body of the rotor. The wheel of Fig. 4 also has an annular shroud plate 26 by which the outer side edges of the blades are connected. Both the arrangement of Fig. 1 and that of Fig. 4 may be considered as providing a separator wheel on each side of the rotor, and both arrangements may be considered as providing a separator wheel having a plurality of circumferentially spaced relatively short radial blades set in axial planes and extending between annular side walls which confine to the spaces between the blades currents of air flowing 'inwardly from the periphery of the wheel. Having the wheels separately removable is of advantage, as will appear, and, as stated above, the arrangement in which the outer side edges of the blades are not connected together is con sidered most desirable.

The axially extending peripheral portions of .the blades of the separator wheels.

the end portions [2 0f the housing are. spaced from the central portion H3 so as to provide between them and the central portion I0 sub stantially continuous annular air inlet slots 30 through which, when the mill is in operation, air is drawn into the mill chamber by s'uction fans 3! in the fan chambers I4. The suction fans draw the air through the separator wheels and thence through annular suction passages 32 between the annular partitions I3 and the hubs of the separator wheels and suction fans and into the fan chambers. flow way from the mill chamber to the suction passages 32 is through the flow spaces between Preferably and as shown, the rotor and the separator wheels and the suction fans are all keyed to and driven by the shaft 15. The hubs of the separately removable separator wheels butt against the body discs of the rotor, and the fan hubs butt against the separator wheel hubs. I

The end walls of the mill chamber are concavely curved as shown at 33 so that, as indicated by the arrows in Fig. '7, the air currents entering through the inlet slots 30 and flowing against these walls are directed inwardly beneath the beater blades and over the materialreceiving peripheries of the separator wheels;

and the inner portions of the edges of the central portion IQ of the housing forming the peripheral wall of the mill chamber .are most desirably flared outwardly to form with the opposite portion of the end wall of the chamber outwardly directed portions of the inlet slots by whichthe entering air currents are confined and directed against the curved end walls.

Inwardly of the concavely curved currentdirecting walls 33 of the chamber, the annular portions 28 of the end walls of the chamber are shaped to the shape of the outer side edges of the blades of the separator wheels and lie very close thereto, so that currents of air flowing inwardly from the periphery of the wheel are confined to the spaces between the blades. Most desirably, the width of the blades of the separator wheels, and therefore the axial width of the interblade spaces between the discs 26 and the wall portions 28 increases in the direction radially inward from the periphery, this increase in width being most conveniently provided by havin the outer side edge of the blades inclined and the portions 28 of the end walls of the mill chamber correspondingly inclined,

as shown. This increase inwardly of the width of the interblade flow spaces of the separator wheels is for the purpose of increasing inwardly the cross-sectional area of the flow spaces to reduce the air drag as the centrifugal force on the particles decreases with decrease in radius.

The words beneath and under, as usedherein in referring to and defining the-position of the periphery of the separator wheels and the location of the air stream with relation to the beater blades of the rotor; are to be under-.

- stood as meaning that the periphery referred to or the air flow referred to are nearer to the 7 axis of rotation than the beater blades in planes normal to the axis of rotation and cutting-the beater blades; and similarly, the words over and above, as used herein in referrin to and defining the position of the beaterblades and the location of the air stream with relation to the periphery of the separator wheels, are to be understood as meaning that the beaterblades or the air stream are farther away from the.

The only axis of rotation than the periphery of the separator wheels in planes normal to the axis of rotation and cutting the peripheries of the separator wheels.

In order to reduce interference with the flow of the air through the interblade spaces of the separator wheels by eddy currents resulting from the entering air current flowing in pathswhich shrink away from the walls of the interblade spaces, forming the so-called contracted veins (vena contracta) the peripheral edge of the discs 26 and especially the angle between the wall 28 and the concavely curved wall 33 are rounded as shown with a radius of curvature which should be greater than and most desirably about of the width of the blades at their outer edge.

The end portions I2 of the housing are so shaped as to provide fan chambers of volute form and have extensions 35 which provide tangential outlet or discharge passages 35. The outer wall of each fan chamber has close to the fan hub several openings 37 through which air may be admitted for reducing the suction through the suction passage 32. The amount of air admitted through these inlet openings of each fan chamber is adjustably controlled, as by an annular damper plate 38 which is secured in adjusted position by screws 39 extending through slots in the plate.

The central portion it of the housing, forming the peripheral wall of the mill chamber, has a feed opening it through which the material to be pulverized is fed, preferably tangentially, into the mill chamber through opening in the liner 2%. The material is supplied to the feed opening through a feed chute i in a part 42 attached to the housing portion W from suitable feeding means which may be mounted on a supporting bracket 43, the feed chute having, in addition to its material inlet opening :34, an air inlet opening 55 for admitting with the material an amount of air which is small compared to the amount of air entering through the annular slots 38.

Since the beater blades 22 act as fan blades, there is a tendency for the air driven against the peripheral wall of the mill chamber to move axially outward in diverging spiral directions toward the ends of the beater blades, as indicated by the arrows in Fig. 7. To assure and increase this spiral outward movement of the air, and with it the pulverized and partly pulverized material, between the edges of the beater blades and the peripheral wall of the chamber, the blades are made wider at the middle and tapering in both directions toward their ends, the outer edges of the blades being straight and the inner edges sloping from the middle portion toward each end'of. the blades. The blades will, because of their tapered form, develop a greater air pressure at the center than at and near their ends.

For convenience in assembling the inside parts of the mill and to provide for convenient access thereto, the central portion it of the housing is. formed in two separable parts Ito and ifib meeting in a diametrical, and preferably horizontal, plane. The lower part liib of the hous ing portion H] has on its two sides mounting extensions 59 which, for supporting the houslog, are seated on and bolted to longitudinal beams of the base frame it; and the upper part iiia is hinged to the lower part Iiib on one side by a hinge bolt 52 connecting hinge arms 53 and 54, respectively, extending one from each part and secured together by a bolt 55, and on the other side the upper part is secured to the lower formed each of two separable parts Fla and no meeting in the same diametrical plane as the parts of the central portion I ll, and each part of each end portion is secured to the corresponding part of the central portion It by screw bolts 60 and spaced therefrom by spacers 6l. The width of the annular air inlet slots 3i) may, obviously, be varied, as may be needed or desirable for 'pulverizing dilferent materials, by the use of shims to supplement the spacers 6| and between the rotor body discs 2s and the separator wheel discs 26.

In the operation of the mill: Because of the fan action of the beater blades and because the tapered blades develop a greater air pressure at the center of the blades than at and near their ends, air in the mill chamber is driven against and circumferentially along the peripheral wall of the mill chamber and caused to move in diverging slightly spiral directions toward the ends of the blades, carrying along with it the material being pulverized, and pulverized and partly pulverized material is constantly discharged from between the beaters and the peripheral chamber wall. As the pulverized and partly pulverized material escapes from the ends of the beaters, moving circumferentially and spirally toward the ends of the mill chamber, it is struck by and becomes diifused in the rapidly moving current of air drawn in through the annular slots at by the suction created by the suction fans 3|. and partly pulverized material diifused therein are directed by the curved end walls 33 and drawn by the suction of the fans inwardly beneath the beater blades. Next, as indicated in Fig. '7, the air streams are divided into two parts. One part flows over the separator wheels and turns radially outwards to enter the hammer path. The other part flows into the material receiving peripheries of the separator wheels. In the separator wheels the pulverized and partly pulverized materia1 in the inwardly flowing air streams is subjected to the separating action of centrifugal force working against the drag of the rapidly moving air, whereby oversize particles are rejected, the acceptably fine material passing on with the air. they leave the separator wheels move circumferentially and spirally outward in air currents induced by the fan action of the beater blades whereby they are carried between the beater blades to be again subjected to the impact pulverizing action of the blades and between the blades and the liner of the peripheral wall of the chamber. The rotor beaters are thus at all times acting both on new material fed into the mill and on the oversize partly pulverized material rejected by the separator wheels. The acceptably fine pulverized material passes on with the air current through the separator wheels and through the passages 32 to the suction fans to be discharged through the tangential discharge passages 36. The fine pulverized material discharged from the suction fan chambers may be separated and collected from the air in which it is suspended in any suitable manner, as by means of cyclone separators.

The rejection of oversize particles from the air streams in the separator wheels results from the fact that, because of the high rotary speed of the separatorwheel blades, centrifugal forces The air streams with pulverized The rejected particles as proportional to their weights are developed in the particles which have entered the wheels at the same time that the particles are subjected to a fluid drag which is proportional to their crosssectional areas and to some extent affected by their shape and surface character. Since the weight of the particles varies as the cube of their diameter; and their cross-sectional area varies as the square of their diameter, the ratio of the centrifugal force acting on the particles to the force of the fiuid drag on the particles increases as the size of the particles increases (this is on the assumption that the particles are substantially spherical and is substantially true for other particles), and this ratio for particles of any given size is maintained constant or substantially so throughout the radial length of the flow spaces since, because of the increase inwardly of the cross-sectional area of the flow spaces, the velocity of the air decreases as it flows in through the spaces so that the fluid drag on the inwardly moving-particles decreases as the centrifugal force acting on the particles decreases. Particles over a certain size, depending on the peripheral speed ofthe separator wheel and the velocity of the entering air, will thus be stopped by centrifugal force and caused to move out of the wheel against the dra of the air.

Table A illustrates the mode of operation of the separator wheels. The figures are based on the assumption that:

('1) Maximum radius of separator wheel is 4 ins.

(2) The separator wheel is turning at 5000 R. P. M.

(3) The specific gravity of the material forming the particles is 2.0.

Table A Velocities, feet per min.

- Relatife P L 1 Diameter particles in partie ar 'iclc vc ocmicrons velocity ity radially n outwards with respect g, 2 with to the axis of 1 T respect to separator the-air 6.0. 1, 955 3, 000 1,045 outward. 5.0. 1,955 2, 500 545 outward. 4.42 l, 955' 1,955 0. 4 4.0. 1,955 1,600 355 inward. 3.0.. l 955 900 L055 inward. 1.0. 1,955 100 1,855 inward.

4.42. 900 l, 055 1.055 outward. 3.0.. 900 000 1.0 900 100 800 inward.

Table B Maximum particle size arah ti e R P M Wl'llCh will pass through separator The blades of the separator wheels ar closely 8 i spaced apart and relatively short radially. Their radial length should most desirably be greater than the greatest distance any particle of the pulverized and partly pulverized material will move inward between the blades during the time the blades move a distance equal to the distance between adjacent blades. The minimum radial length for any spacing of the blades will thus depend on the speed at which the wheels rotate and the flow rate of the air entering between the blades. For a mill in which the separator wheels are about 9" in diameter and have blades wide'at the periphery and are driven at about 6,000 R. P. M. and with about 600 or 700 cubic feet of air passing through the two separator wheels per minute, the blades may be about long and be spaced about 2 apart. With substantially more air, or with th blades spaced further apart, the blades should be longer. If the blades of a wheel are very closely spaced and correspondingly short, good separation may be obtainedwithouth'aving the side walls relatively inclined to cause the cross-sectional area of the interblade spaces to increase inwardly.

The fineness to which the material is pulverized by the rotor will, of course, vary according to the speed of the beater blades and the time the material remains in the mill and also on the character of the material. The fineness of the product delivered from the mill, however. depends on what size particles are allowed to pass through the separator wheels and what size particles are rejected by the separator wheels and returned to the beater blades, and this varies.

according to the peripheral speed of the wheels and the velocity of the air entering the wheels and the density of the material. air Velocity, the finer will'be the product; and the greater the peripheral speed of the wheels and, therefore, the greater the centrifugal force exerted on the particles, the finer will be the product. Therefore, if, with other conditions remaining constant, wider separator wheels are used, or the air inlet opening 37 are opened, or

suction fans of less capacity are used, a finer product will be obtained and if, other conditions re- 1, 2 and 3, the separator wheels and the partition members l3 are readily removable, and separator wheels of different diameters within limits, or having blades of different widths, i. e. dimension axially of the mill, within limits, and partition members conforming to the new wheels, may be installed in the mill. In this way the maximum grain size of the product of the mill may be varied within a considerable range, and the range of variation may be increased by varying the suction on the separator wheels, as by adjustment of the air inlet openings 3! or by using difierent suction fans or by driving the fans at different speeds.

Because of the double ended, or duplex, character of the mill shown, and of the possibility of obtainingproduct of different degrees of fineness by replacement of parts or adjustment as above pointed out, it is possible with this mill to obtain at the same time two products of difierent degrees of finenessby having one end of the mill itted and adjus ed o a pr uct of one-degree The lower the i of fineness and the other end for a product of a different degree of fineness.

The outer side edges of the separator wheel blades should, as stated, be closely spaced from the walls 28, the edges of the blades and the walls being conformed to each other. This is necessary in order to avoid leakage of oversize particles past the wheels. The clearance between the blades and the walls may be greater when the wall is inclined as shown than would be permissible if the wall were normal to the wheel axis. The permissible clearance will vary for wheels having blades of diiferent lengths and Widths. In a 12 mill such as the one shown, separator wheels having blades wide at the periphery and wheels having blades wide at the periphery, and 1" long in both cases, have been used with a clearance of approximately .030". With this clearance with these blades, leakage of oversize particles past the wheels was hardly noticeable. With a clearance of about .06" with the blades, there was a slight passage of slightly oversize particles. With wider and longer blades the clearance may, it is believed, be substantially increased, the permissible clearance depending on the width and length of the blades and the speed of the wheel and the velocity of the entering air. For blades about to A" Wide at the periphery and of suitable length as stated in column 8, the clearance should be less than of the blade width, and for best results not more than about 10%, and the ratio of clearance to blade width should decrease as the blade width increases.

Due apparently to the high speed circumferential movement given to the pulverized and partly pulverized material by the beater blades, some of the larger particles were found to move out through the slots against the inflowing air. This has been largely reduced by making the width of the liner 24 of the peripheral Wall of the mill chamber somewhat greater than the axial length of the beater blades, as shown, so that the edges of the liner extend substantially beyond the ends of the blades. Even then, however, a very few of the larger particles were found to escape from the slots, and to hold these escaping particles an annular chamber 65 is provided for each of the slots which serves as a trap for retaining such particles. These chambers have a wall 55 against which the escaping particles strike, and have their air inlet opening protected by a screen 6! which serves not only to retain the particles in the trap but also to prevent the entrance with the inflowing air of whatever might otherwise be drawn into the slot.

The new mill is of high efiiciency, as evidenced by its low power consumption compared to its capacity and fineness of product; it will deliver a product of extreme fineness having only a very small fractional percentage of slightly oversize particles; it has high production capacity for its size; it can with most materials be operated continuously for long periods of time without clogging by accumulation of ground material in corners or against dead walls; it may be used to produce at the same time product of two different degrees of fineness; and temperature rise during operation of the material being ground and of mill parts is comparatively low; and it can be used for pulverizing materials of widely diferent characteristics and properties.

As an example of the performance of the new mill, av mill such as shown having a mill chamber 12" in diameter and separator wheels 9" in diameter with blades wide at the peripheral edge, with the rotor, separator wheels and suction fans driven at 6000 R. P. M. and a little more than 600 cu. ft. of air passing through the two separator wheels per minute, has delivered over 400 lbs. of sugar per hour of which 99.96% will pass through a 325 mesh screen (i. e. below 40 microns diameter), the power consumption having been under 15 H. P. With separator wheels having blades wide at the periphery, still finer sugar has been produced, and in pulverizing Ti-Cal (white pigment) and anhydrous CuSOi, product of considerably less than quarter the above particle size is obtained.

The arrangement of the mill shown by Fig. l to 3, which has two suction fans both mounted within the mill housing and driven by the rotor shaft, has many advantages. It is sometimes desirable, however, to provide for conveniently adjustably varying the suction through the mill chamber and separator wheels for varying the fineness of the product or for the pulverizing of diiTerent materials. Adjustment of fan speed will give a greater range of variation of the suction through the mill chamber than is practically obtainable by means of the damper plates 38, and the simplest and probably the best way to provide for fan speed adjustment is by having a single independently driven suction fan outside the mill housing.

Figs. 5 and 6 illustrate the use of a single independently driven suction fan for maintaining the air flow through the mill chamber and separator wheels of a double ended or duplex mill according to the invention. Referring to said figures, the mill housing I0 and its contained operating parts may be the same as shown by Figs. 1 and 3 except that, instead of the fan chambers IA of Figs. 1 and 2 and the fans 3! capable of maintaining the desired air flow through the mill chamber and separator wheels, the fan casing part of the end portions l2 of the housing of the mill illustrated by Figs. 5 and 6 are made much smaller, providing relatively small volute casings in each of which, most desirably, there is mounted on the rotor shaft [5 a relatively small fan 10 which is merely for the purpose of acting as a booster to overcome any formation of eddy currents and aid the free flow through the casing of the air containing the acceptably fine pulverized material. From the tangential discharge outlet 35 of each of these small volute casings, a conveying pipe H leads to the intake opening of a suction fan 12 on a shaft 13 driven by any suitable variable speed driving motor, or, as it might be, from a constant speed motor driving through a variable speed gear.

By the term separator wheel as used in some of the claims is meant a wheel having a plurality of circumferentially arranged radially extending relatively short flow spaces open at the periphery in which pulverized material carried by air flowing at high velocity inwardly therethrough when the wheel is rotating at high speed is subjected to the separating action of centrifugal force working against the drag of the air.

What is claimed is:

1. In an impact pulverizing mill, a housing comprising a mill chamber'portion which provides a cylindrical mill chamber and which is formed of two separable parts meeting in a plane diametrical of the mill chamber and an end portion formed of two parts each of which is secured ll to one of the parts of the mill chamber portion and spaced therefrom to leave a substantially continuous annular air inlet slot between the mill chamber portion and the end portion, an

annular partition piece which fits within the end portion of the housing and forms together with a part of the end portion an end wall of the mill chamber, the end portion of the housing forming with the annular partition piece a fan -chamber, a drive shaft extending through the housing, a rotor having a plurality of circumferentially spaced beater blades and a separator wheel at one side of the rotor both fast on said shaft, and a suction fan in said fan chamber for drawing air through said air inlet slot and through said separator wheel.

2. In an impact pulverizing mill, a housing comprising a central portion which provides a cylindrical mill chamber and two end portions each of which is secured to the central portion and spaced therefrom to leave a substantially continuous annular air inlet slot between the central portion and each of the end portions, two

, annular partition pieces which fit within the end 7 portions of the housing and each of which forms together with a part of one of the end portions an end wall of the mill chamber, each end portion of .the housing forming with its annular partition piece a volute fan chamber having a tangential discharge outlet, a drive shaft extending through the housing, a rotor fast on said shai't having a plurality of circumferentially spaced beater blades, two separator wheels one on each side of the rotor both fast on said shaft, and two suction fans one in each of said fan chambers for drawing air through said air inlet slots and through said separator wheels.

3. In an impact pulverizing mill, a housing comprising a central portion which provides a cylindrical mill chamber and which is formed of two separable parts meeting in a plane diametrical of the mill chamber and two end portions each formed of two parts each of which is se cured to one of the parts of the central portion and spaced therefrom to leave a substantially continuous annular air inlet slot between the central portion and each of the end portions, two annular partition pieces which fit within the end portions of the housing and each of which forms together with apart of one of the end portions an end wall of the mill chamber, each end portionof the housing forming with its annular partition piece a volute fan chamber having a tangential discharge outlet, a drive shaft extending through the housing mounted in bearings outside the housing, a rotor fast on said shaft having a plurality of circumferentially spaced beater blades, two separator wheels one on each side of the rotor both fast on said shaft, and two suction fans one in each of said fan chambers for drawing air through said air inlet slots and through said separator wheels.

4. An impact pulverizing mill as claimed in claim 3, in which the annular partition pieces are interlocked with the end portions when the two parts of the end portions are positioned together and are removable when the two parts of the end portions are separated.

5. In an impact pulverizing mill, a housing comprising a central portion which provides a cylindrical mill chamber and two end portions each providing a fan chamber and each of which is secured to the central portion and spaced therefrom to leave a substantially continuous 12 air inlet slot between the central portion and each of the end portions, a drive shaft extending through the housing, a rotor having a plu rality of circumferentially spaced beater blades in the mill chamber fast on said shaft, a separator wheel on each side of the rotor fast on said shaft, and a suction fan in each of the fan chambers for drawing air through said air inlet than the axial dimension of the chamber and the end of the chamber surrounding the hammer ends being rounded to cooperate in creating the helical air current, means for admitting a current of air into the rounded end of said chamber substantially tangentially to the direction of movement of the air in the chamber about the axis of said helix to aid such helical movement,

and means for withdrawing an equal amount of air, from said chamber.

7. A pulverizer as claimed in claim 6 in which the air-admitting means is adjacent the point at which the helical current of air is moving radially inwardly and the air-withdrawal means is adjacent the point at which the helical current of air begins to move radially outwardly;

8. An impact pulverizer comprising a rotor,,a series of hammers mounted peripherally thereon, said rotor including a central portion having therein two parallel sets of radially extending air discharge passages having a total axial width less than the axial width of the hammer path, an annular casing surrounding said hammers peripherally and laterally, means for admitting material to be pulverized and air into said casing, the air entering the casing beyond the ends of the hammers, said casing also extending inwardly toward each side of the rotor adjacent to the outer ends of said air passages to compel air discharged from the casing to flow through said discharge passages.

9. An impact pulverizer comprising a rotor a series of laterally-extended hammers mounted peripherally thereon; said rotor including a central portion having therein two parallel sets'of radially-extending air discharge passages having a total axial width less than the axial width of the hammer path, an annular casing closely surrounding said hammers peripherally while extending laterally a considerable distance from the ends of the hammers, means for admitting material to be pulverized and air into said casing, the air entering the casing beyond the ends of the hammers, said casing also extending inwardly toward eachside of the rotor adjacent the outer ends of said air passages to compel air discharged from the casing to flow through said discharge passages wherein the over-size particles are thrown outwards by centrifugal force into the path of the hammers.

10. An impact pulverizing mill, comprisinga housing providing an annular mill chamber, a high speed rotor, a plurality of circumferentially spaced beater blades within said chamber arid mounted on said rotor, said blades terminating short of the end of said chamber to permit air placed in circulation by the blades to escape in an axial direction, means for introducing material to be pulverized into the path of sai blades, a high-speed separator wheel mounted coaxially with the rotor and with its periphery forming a part of the inner wall of said chamber having a plurality of circumferentially arranged relatively short radial flow passages substantially closed except at their inner and outer peripheries, means for maintaining a strong flow of air into said chamber and then radially inwards through said flow passages in said separator wheel, the inner wall of said casing and the periphery of said wheel being spaced radially away from the beater blades with the said passages directly beneath a part of said blades, said chamber having a curved end wall for deflecting the air leaving the blades radially inwardly, then under the blades and substantially axially toward the entrances to the said radial flow passages to permit a part of the air from the hammers to re-enter the hammer path and another part to enter the passages in the separator wheel, where the over-size is rejected and thrown into that part of the air which re-enters the hammer path while the acceptably fine material passes through the separator with the other part of the air.

11. An impact pulverizing mill as in claim 10 in which the mill chamber is provided with a substantially continuous annular air inlet slot through its outer periphery adjacent to but axially outside of the path of the beater blades.

12. An impact pulverizing mill having an annular hammer chamber roughly elliptical in cross-section on planes passing through the axis of the annulus, means for introducing material to be pulverized into said chamber, a single set of circumferentially spaced hammers mounted for rotation within said chamber adjacent the outer wall thereof, the inner wall of the chamber being spaced away from the inner margins of the hammers sufiiciently to provide an axially extending air-space of considerable radial depth therebeneath, the walls of said chamber having two annular openings, one for the admission of air and the other located in said inner wall for the discharge of dust-laden air, rotating, radially-passaged centrifugal separating means in said last-mentioned opening for radially outwardly rejecting the over-size, and means for maintaining a strong flow of air into said chamber through said admission opening and out of said chamber by radially inward passage through said centrifugal separating means.

13. An impact pulverizer comprising a chamber, means for admitting material to be pulverized and air into said chamber, a rotor, a series of hammers thereon travelling around a circular path within said chamber to project material to be broken outwardly against the surrounding chamber wall and to place the air in the hammer path in rotation and thereby draw air radially outwardly into said path and push it laterally outwards from such path, the strip of Wall receiving such projected material being imperforate except for a material-feed inlet, means for deflecting the air which has been pushed laterally outwardly by the movement of the hammers, first radially inwards and then laterally and axially toward the central plane of the rotor where part of it is drawn radially outwards by the rotation of the hammers for re-entry into the path of the latter, centrifugal rotating,

radially-passaged separating means opening into the radially innermost wall of the chamber for discharging air and acceptably fine material from the chamber towards the axis of the rotor adjacent the point at which a part of suchdefiected air is drawn radially outwards for reentry into the hammer path, and means for maintaining a strong flow of air into and through said chamber, then into and radially inwardly through said separating means.

14. An impact pulverizing mill having an annular hammer chamber roughly elliptical in cross-section on planes passing through the axis of the annulus, means for introducing material to be pulverized into said chamber, a single set of circumferentially spaced hammers mounted for rotation within said chamber adjacent the outer wall thereof, one wall of said chamber having an annular opening for the admission of air, the inner wall of the chamber being spaced away from the inner margins of the hammer sufficiently to provide an axially extending airspace of considerable radial depth and having a radial inwardly-extending annular opening for the discharge of dust-laden air, centrifugal rotating, radially-passaged separating means in said last-mentioned opening for the rejection of oversize particles, and means for maintaining a strong flow of air into and through said chamber, then into and radially inwardly through said separating means.

WILLIAM M. SHELDON.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date Re.16,229 Blyth Dec. 15, 1925 1,221,144 Crites Apr. 3, 1917 1,796,545 Taylor Mar. 17, 1931 2,013,800 Daniels Sept. 10, 1935 2,042,042 Frisch May 26', 1936 2,359,911 Grindle Oct. 10, 1944 2,355,784 Dondlinger Aug. 15, 1944 2,361,758 De Fligue Oct, 31, 1944 FOREIGN PATENTS Number Country Date 771,372 France July 23, 1934 259,632 Germany May 10, 1913 203,632 Great Britain Sept. 13, 1923' 444,684 Great Britain Mar. 25, 1936 

